The development of the human body; a manual of human embryology . Fig. 81.—LongitudinalSection through theNail Area in an Em-bryo of 17 CM. ep, Eponychium; «, nailsubstance; nb, nail bed;sc, stratum corneum;sp, sole plate.—(Oka-mum ) The first appearance of the nail areasat the tips of the digits as describedby Zander has not yet been con-firmed by later observers, but the mi-gration of the areas to the dorsal sur-face necessitated by such a locationof the primary differentiation affords THE HAIR. 167 an explanation of the otherwise anomalous cutaneous nerve-supply of the nail areas in the adu
The development of the human body; a manual of human embryology . Fig. 81.—LongitudinalSection through theNail Area in an Em-bryo of 17 CM. ep, Eponychium; «, nailsubstance; nb, nail bed;sc, stratum corneum;sp, sole plate.—(Oka-mum ) The first appearance of the nail areasat the tips of the digits as describedby Zander has not yet been con-firmed by later observers, but the mi-gration of the areas to the dorsal sur-face necessitated by such a locationof the primary differentiation affords THE HAIR. 167 an explanation of the otherwise anomalous cutaneous nerve-supply of the nail areas in the adult, this being from thepalmar (plantar) nerves. The Development of the Hairs.—The hairs begin todevelop at about the third month and continue to beformed during the remaining portions of fetal life. They. Fig. 82.—The Development of a , Cylindrical cells of stratum mucosutn; kj, wall of hair follicle; m,mesoderm; tnu, stratum mucosum of epidermis; p, hair papilla; r,root of hair; s, sebaceous gland.—{Kallmann.) arise as solid cylindrical downgrowths, projecting ob-liquely into the subjacent dermis from the lower surfaceof the epidermis. As these downgrowths continue toelongate, they assume a somewhat club-shaped form (), and later the extremity of each club moulds itself 168 THE DEVELOPMENT OF THE HUMAN BODY. over the summit of a small papilla which develops fromthe dermis (Fig. 82). Even before the dermal papillahas made its appearance, however, a differentiation ofthe cells of the downgrowth becomes evident, the centralcells becoming at first spindle-shaped and then undergo-ing a keratinization to form the hair shaft, while the moreperipheral ones assume a cuboidal form and constitutethe lining of the hair follicle. The further growth of thehair takes place by the add
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
